The present invention relates to rotary rock bits, and, more in particular, to an improved rotary rock bit of the friction bearing type and a method for manufacturing the improved rock bit.
Rotary rock bits are used in earth boring, for example, for access to formations containing petroleum values. Typically, each rock bit has one or more legs or segments, with each segment having a journal rotatably carrying a cone cutter. The axis of the journal lies at an acute angle to both the vertical and the horizontal. Each cone cutter is compressively loaded between the bottom of the journal by the weight of the drill string and the radial surface at the bottom of the hole being drilled.
These compressive loads can be very high because of the weight of the drill string on the cutters, for the drill string can be thousands of feet in length. A friction type journal bearing can withstand these high loads better than anti-friction ball or roller bearings, so long as the friction bearing is continuously lubricated. A sealed lubricant system in communication with the friction bearing provides the lubrication, as described in U.S. Pat. No. 3,917,028. The lubrication system includes a variable volume reservoir that expands and contracts in response to pressure differentials between the lubricant and the drilling environment. The system then can adjust for changes in pressure due to drilling mud head and expansion and volatilization of the lubricant at the high temperatures of deep holes.
To improve wear of the journal bearing surface on the journal, it has been the practice to provide a hard bearing metal wear pad on the journal on that portion which sustains heavy loading. It has also been the practice to offset the axis of curvature of the wear pad from the axis of the journal to avoid squeezing a seal too much.
Ball bearings tracking in a race at the journal and on the cone cutter secure the cutter to the journal. To avoid Brinneling the race of the journal, the race and adjoining lands have been hardened by carburization and subsequent heat treatment. A wear resistant surface adjacent the O-ring seal between the journal and the leg has also been provided by carburizing and heat treatment.
The journal has a thrust face lying normal to the axis of the journal between a nose of the journal and the ball race to take the thrust of the cone cutter. Thrust may also be taken at the end of the nose of the journal which lies close to the axis of the bit. These areas, being subject to wear, have also been carburized and heat treated, or hardfaced.
In the past, the selective carburization and heat treatment has been arduous, requiring much handwork. The machined areas of the faying surfaces, where the segment abuts neighboring segments, and journal as well as other areas were stopped off. Stop off was either by ceramic based paint or by electroplated masking material. Non stopped off areas were carburized. Carburization was at an elevated temperature, for a long period of time, and in a carburizing atmosphere. For example, a carburizing atmosphere of 0.6% to 0.8% carbon potential for 15 hours and at about 1700.degree. F. was used in carburizing a segment to obtain 0.040 to 0.060 inches depth of carburization. The part was then slowly cooled so as to avoid hardening the carburized surfaces. When the ceramic based stop-off was used, this material was then removed by hand sandblasting in the selected areas of where the grease reservoir and nozzle were to be formed. The part was then finish machined. Finish machining includes development of the grease reservoir and the nozzle. After finish machining, the part was heat treated by heating, quenching and tempering. Thereafter, all remaining ceramic stop-off was removed and the parts, wear pad and other journal wear surfaces were finished ground.
Quite obviously, the selective carburization process used in the past consumes both time and money.
In the selective carburization technique it was thought that some areas required deep carburization to withstand imposed loads, for example, the roller races. Deep carburization in other areas would be bad, however, because those areas are not thick enough to provide a meaningful soft and tough core. Examples of such thin areas include the ball flanges and shirttail. The faying surfaces and areas adjacent thereto must be maintained at a low carbon level because of the adverse effect excessive carbon has on the integrity of the welds at the faying surfaces. For example, cracks could develop creating leak paths. For those reasons carburization in rock bits was thought to require the selective area technique described.